Certain fluorine containing polymers (also referred to as “fluoropolymers”) are known to have desirable properties such as good chemical resistance, excellent release, good heat resistance and electrical insulation. The excellent release properties have lead to fluoropolymers, particularly fluoropolymer powders that are melt-flowable, to be useful in coating food preparation articles such as, for instance, fry pans, rice cookers, grills, and bakeware. Fluoropolymers have also found use in numerous industrial applications such as fuser rolls or belts for copiers and printers, vessels, and chemical processing reactors.
Fluoropolymers may be coated onto substrates by a number of conventional methods, for instance, by dispersion coating or by powder coating. When powder coating a substrate, powder particle size distribution can influence the powder delivery system, the charging system and the final film characteristics. For feed hopper delivery, steady, uniform fluidization of the powder is required to achieve a consistent flow through the pumps and out the spray device. If the concentration of fine particles is too high, powder coating material tends to clump together and resist fluidization. Higher air pressure may alleviate some of the clumping, but this can cause geysering in the fluidization bed. By geysering is meant when the surface of the fluidized powder becomes inconsistent with large eruptions from the bed surface and dead spots. This condition may cause air to surge and deliver inconsistent amounts of powder to the spray device. This may result in inconsistent film thickness and possibly surface blemishes. If severe, the spray device may spit a large volume of powder onto the surface and leave a blotchy mound of powder. The higher moisture content of powders with high concentrations of fine particles may contribute to additional clumping.
Particle size can also influence the charging system, that is, particle size may affect the electrostatics of powder delivery. Back-ionization related to fine particles can contribute to surface disruptions at thinner films. Larger particles, on the other hand, are more likely to have straight line motion and be affected only by strong electrostatic force lines. These properties can affect the transfer efficiency of a powder coating.
Film characteristics, such as thickness and smoothness, are also affected by particle size. In general, coarser powders (larger particle size) will build thicker films, while finer powders (smaller particle size) will provide thinner films. Large particles, particularly those with high molecular weight, take longer to melt and flow and may not completely level out during processing, leading to rough surfaces. If the powder is too coarse, it may cause excessive texture called “orange peel.” An excess of fines, on the other hand, may cause a film to reach the self-limiting point faster and start back ionization, resulting in “electrostatic orange peel.”